[WIP] WASM: share empty child arrays

[peaBerberian]

Preamble (repeated for the current wave of WASM PRs)

I'm doing several PR related to our optional WebAssembly DASH parser.

This parser was initially specifically optimized for DASH MPD with huge <SegmentTimeline> elements (several MB) for some of Canal+'s own contents.

Testing it lately, it is still the main scenario where it clearly shines against our default JS parser.
Since that time however, the DASH spec and DASH packagers have evolved a lot and those bigger MPDs are now very rarely encountered. At Canal+, this led us back to advise the usage of our simpler default JS parser to applications. Yet we still continued maintaining in a best-effort way our WASM parser, as we know some external applications continue to rely on it.

It's been a few years since we've really dived deeply into it however, and with the new tools at our disposal now (notably LLMs, particularly adapted here as we've two pieces of code - the JS and WASM parsers - which are supposed to give the same output), there's some areas for a few quick wins in terms of conformance, in terms of testing against regressions, and in terms of performance.

What this PR does

Reuse the same "empty" array reference. As we might have a lot of it in our parser. (actually allocate a new array for non-empty).

We'll see in CI if there's a point

[github-actions]

✅ Automated performance checks have passed on commit 5887cb48171b016402bd14ad2eed713a9bc694f7 with the base branch dev.

Details

Performance tests 1st run output

No significative change in performance for tests:

Name	Mean	Median
DASH JS parser - regular manifest (20 parses)	29.57ms -> 29.69ms (corrected: 0.076ms, A/A bias: -0.193ms, z: 0.12028)	29.70ms -> 29.70ms (corrected: 0.075ms, A/A bias: -0.175ms, z: 0.11547)
DASH WASM parser - regular manifest (20 parses)	16.43ms -> 15.25ms (corrected: 1.350ms, A/A bias: -0.168ms, z: 0.10585)	14.50ms -> 14.90ms (corrected: -0.225ms, A/A bias: -0.075ms, z: 0.49075)
DASH JS parser - many AdaptationSets (20 parses)	15.90ms -> 15.50ms (corrected: 0.258ms, A/A bias: 0.137ms, z: 0.34641)	15.20ms -> 15.05ms (corrected: 0.075ms, A/A bias: 0.025ms, z: 0.60622)
DASH WASM parser - many AdaptationSets (20 parses)	17.04ms -> 16.46ms (corrected: 0.662ms, A/A bias: -0.078ms, z: 2.35751)	16.50ms -> 15.90ms (corrected: 0.600ms, A/A bias: 0.025ms, z: 3.72872)
DASH JS parser - short SegmentTimeline (100 parses)	4.24ms -> 4.39ms (corrected: -0.173ms, A/A bias: 0.023ms, z: 3.03590)	4.00ms -> 4.30ms (corrected: -0.275ms, A/A bias: 0.000ms, z: 3.04071)
DASH WASM parser - short SegmentTimeline (100 parses)	3.61ms -> 3.57ms (corrected: 0.034ms, A/A bias: 0.007ms, z: 0.94782)	3.40ms -> 3.40ms (corrected: -0.100ms, A/A bias: 0.050ms, z: 1.70799)
DASH JS parser - medium SegmentTimeline (4 parses)	5.55ms -> 5.62ms (corrected: -0.065ms, A/A bias: -0.013ms, z: 1.03442)	5.40ms -> 5.50ms (corrected: -0.050ms, A/A bias: 0.000ms, z: 1.50111)
DASH WASM parser - medium SegmentTimeline (4 parses)	3.13ms -> 3.07ms (corrected: 0.092ms, A/A bias: -0.039ms, z: 0.98631)	3.10ms -> 3.00ms (corrected: 0.025ms, A/A bias: 0.000ms, z: 1.43856)
DASH JS parser - eight-hour SegmentTimeline (1 parse)	60.17ms -> 60.14ms (corrected: 1.633ms, A/A bias: -1.598ms, z: 0.24056)	55.55ms -> 56.75ms (corrected: -1.625ms, A/A bias: -0.825ms, z: 0.24056)
DASH WASM parser - eight-hour SegmentTimeline (1 parse)	32.62ms -> 31.64ms (corrected: 1.311ms, A/A bias: -0.330ms, z: 0.26943)	28.80ms -> 29.20ms (corrected: -0.075ms, A/A bias: -0.050ms, z: 0.26943)
loading	22.98ms -> 20.39ms (corrected: 2.580ms, A/A bias: 0.011ms, z: 0.10585)	16.70ms -> 16.80ms (corrected: 0.025ms, A/A bias: -0.025ms, z: 0.43301)
seeking	516.13ms -> 548.52ms (corrected: -59.997ms, A/A bias: 27.608ms, z: 1.37602)	1004.60ms -> 1004.90ms (corrected: -0.525ms, A/A bias: 0.350ms, z: 1.02480)
audio-track-reload	22.54ms -> 22.63ms (corrected: 0.123ms, A/A bias: -0.212ms, z: 0.09141)	22.30ms -> 22.60ms (corrected: 0.025ms, A/A bias: -0.025ms, z: 0.13953)
loading over active content	13.39ms -> 12.77ms (corrected: 0.790ms, A/A bias: -0.176ms, z: 0.68320)	12.60ms -> 12.60ms (corrected: 0.075ms, A/A bias: -0.100ms, z: 0.68320)
large multi-period manifest	51.83ms -> 51.75ms (corrected: 0.051ms, A/A bias: 0.036ms, z: 0.08660)	50.30ms -> 51.45ms (corrected: 0.350ms, A/A bias: -0.325ms, z: 0.08660)
cold loading multithread	37.96ms -> 37.38ms (corrected: 3.206ms, A/A bias: -2.627ms, z: 2.65581)	37.20ms -> 36.75ms (corrected: 0.475ms, A/A bias: -0.075ms, z: 2.87713)
seeking multithread	881.50ms -> 841.65ms (corrected: 4.709ms, A/A bias: 35.138ms, z: 0.05774)	1005.60ms -> 1005.50ms (corrected: -0.025ms, A/A bias: 0.225ms, z: 0.89489)
audio-track-reload multithread	22.88ms -> 22.68ms (corrected: 0.302ms, A/A bias: -0.103ms, z: 1.05366)	22.70ms -> 22.40ms (corrected: 0.325ms, A/A bias: -0.050ms, z: 2.03997)
hot loading multithread	14.41ms -> 14.22ms (corrected: 0.296ms, A/A bias: -0.107ms, z: 2.12657)	14.20ms -> 14.10ms (corrected: 0.325ms, A/A bias: -0.150ms, z: 2.29497)

[github-actions]

✅ Automated performance checks have passed on commit af9230eda33df8059fbd2efff651bc3ef0169888 with the base branch dev.

Details

Performance tests 1st run output

No significative change in performance for tests:

Name	Mean	Median
DASH JS parser - regular manifest (20 parses)	41.82ms -> 42.05ms (corrected: 0.148ms, A/A bias: -0.375ms, z: 0.25981)	40.70ms -> 39.40ms (corrected: -0.350ms, A/A bias: 0.175ms, z: 0.26943)
DASH WASM parser - regular manifest (20 parses)	19.44ms -> 19.84ms (corrected: -0.878ms, A/A bias: 0.481ms, z: 1.45300)	17.40ms -> 17.80ms (corrected: -0.600ms, A/A bias: 0.100ms, z: 1.51554)
DASH JS parser - many AdaptationSets (20 parses)	24.89ms -> 24.71ms (corrected: 0.163ms, A/A bias: 0.021ms, z: 0.71207)	23.80ms -> 23.70ms (corrected: 0.150ms, A/A bias: -0.025ms, z: 0.41377)
DASH WASM parser - many AdaptationSets (20 parses)	17.95ms -> 17.94ms (corrected: 0.003ms, A/A bias: 0.009ms, z: 0.13472)	16.70ms -> 16.80ms (corrected: 0.125ms, A/A bias: -0.075ms, z: 0.07217)
DASH JS parser - short SegmentTimeline (100 parses)	6.84ms -> 7.02ms (corrected: -0.170ms, A/A bias: -0.006ms, z: 1.48668)	6.60ms -> 6.80ms (corrected: -0.100ms, A/A bias: -0.050ms, z: 2.19393)
DASH WASM parser - short SegmentTimeline (100 parses)	5.62ms -> 5.59ms (corrected: 0.016ms, A/A bias: 0.009ms, z: 0.19726)	5.40ms -> 5.40ms (corrected: 0.000ms, A/A bias: 0.000ms, z: 0.64471)
DASH JS parser - medium SegmentTimeline (4 parses)	9.08ms -> 8.94ms (corrected: 0.151ms, A/A bias: -0.009ms, z: 0.92376)	8.80ms -> 8.70ms (corrected: 0.000ms, A/A bias: 0.025ms, z: 0.68801)
DASH WASM parser - medium SegmentTimeline (4 parses)	5.24ms -> 5.34ms (corrected: -0.135ms, A/A bias: 0.031ms, z: 2.14101)	5.20ms -> 5.30ms (corrected: -0.100ms, A/A bias: 0.000ms, z: 0.91895)
DASH JS parser - eight-hour SegmentTimeline (1 parse)	88.39ms -> 91.09ms (corrected: -2.548ms, A/A bias: -0.152ms, z: 0.66876)	82.95ms -> 85.80ms (corrected: -8.075ms, A/A bias: 1.725ms, z: 0.66876)
DASH WASM parser - eight-hour SegmentTimeline (1 parse)	51.88ms -> 52.04ms (corrected: 0.316ms, A/A bias: -0.471ms, z: 0.17321)	46.70ms -> 46.50ms (corrected: -0.125ms, A/A bias: -0.800ms, z: 0.17321)
loading	27.28ms -> 27.10ms (corrected: -0.957ms, A/A bias: 1.133ms, z: 0.25981)	22.40ms -> 22.30ms (corrected: 0.025ms, A/A bias: 0.125ms, z: 0.25500)
seeking	134.08ms -> 141.88ms (corrected: -12.836ms, A/A bias: 5.038ms, z: 0.92376)	11.40ms -> 11.90ms (corrected: -1.050ms, A/A bias: 0.350ms, z: 2.55477)
audio-track-reload	34.12ms -> 34.14ms (corrected: -0.047ms, A/A bias: 0.021ms, z: 0.27905)	33.50ms -> 33.95ms (corrected: 0.125ms, A/A bias: -0.250ms, z: 0.15877)
loading over active content	20.49ms -> 20.59ms (corrected: -0.222ms, A/A bias: 0.129ms, z: 0.32717)	19.35ms -> 19.65ms (corrected: -1.150ms, A/A bias: 1.050ms, z: 0.32717)
large multi-period manifest	80.22ms -> 79.88ms (corrected: -0.459ms, A/A bias: 0.795ms, z: 0.18283)	73.95ms -> 73.30ms (corrected: 4.825ms, A/A bias: 0.150ms, z: 0.18283)
cold loading multithread	52.70ms -> 52.02ms (corrected: 0.496ms, A/A bias: 0.184ms, z: 1.13546)	51.70ms -> 51.00ms (corrected: 0.625ms, A/A bias: -0.000ms, z: 3.47854)
seeking multithread	73.90ms -> 116.43ms (corrected: -80.035ms, A/A bias: 37.507ms, z: 2.56921)	13.50ms -> 13.70ms (corrected: -0.325ms, A/A bias: 0.175ms, z: 2.07846)
audio-track-reload multithread	31.12ms -> 31.08ms (corrected: 0.537ms, A/A bias: -0.500ms, z: 1.44338)	30.30ms -> 30.30ms (corrected: 0.175ms, A/A bias: -0.250ms, z: 0.45226)
hot loading multithread	21.29ms -> 21.19ms (corrected: -0.272ms, A/A bias: 0.376ms, z: 0.62546)	20.40ms -> 20.40ms (corrected: -0.100ms, A/A bias: 0.075ms, z: 0.72169)